What is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light.
In short, our galaxy was named because of the way the haze it casts in the night sky resembled spilled milk. This name is also quite ancient. It is translation from the Latin “Via Lactea“, which in turn was translated from the Greek for Galaxias, referring to the pale band of light formed by stars in the galactic plane as seen from Earth.
Persian astronomer Nasir al-Din al-Tusi (1201–1274) even spelled it out in his book Tadhkira: “The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color.”
Astronomers had long suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many, many more stars in the sky, and that all of the ones that we can see are a part of the Milky Way.
The Milky Way looks brightest toward the galactic center, in the direction of Sagittarius. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies near the galactic plane. The Milky Way has a relatively low surface brightness due to the gases and dust that fills the galactic disk. That prevents us from seeing the bright galactic center or from observing clearly what is on the other side of it.
If you could travel outside the galaxy and look down on it from above, you’d see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have 2 spiral arms, called Scutum–Centaurus and Carina–Sagittarius.
The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region. However, the existence of these arms has been determined from observing parts of the Milky Way – as well as other galaxies in our universe – and are not the result of seeing our galaxy as a whole.
In truth, all pictures that depict our galaxy are either artist’s renditions or pictures of other spiral galaxies. Until recently, it was very difficult for scientists to gauge what the Milky Way looks like, mainly because we’re embedded inside it. If you had never been outside of your own house, you wouldn’t know what it looked like from outside. But you’d get a sense by looking at the interior and comparing it to other houses in the neighborhood.
From ongoing surveys of the night sky with ground-based telescopes, and more recent missions involving space telescopes, astronomers now estimate that there are between 100 and 400 billion stars in the Milky Way. They also think that each star has at least one planet, which means there are likely to be hundreds of billions of planets in the Milky Way – and at least 17 billion of those are believed to be the size and mass of the Earth.
Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way, and about 27,000 light years from the galactic core. At the heart of the Milky Way is a super-massive black hole, just like all of the other galaxies, known as Sagittarius A*. This monster is more than 4 million times the mass of the Sun and is believed to have a diameter of 44 million kilometers.
Our Sun takes about 240 million years to orbit the Milky Way once, in what is known as a galactic year (or cosmic year). Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made an estimated 18-20 trips around in its entire life. By this reckoning, the birth of our Sun took place 18.4 galactic years ago, and the Universe itself was created approximately 61 galactic years ago.
The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter, which accounts for some 90% of its mass. Nobody knows precisely what dark matter is, but its mass has been inferred by observations of how fast the galaxy rotates and other general behaviors. More importantly, it is believed that this mass helps keep the galaxy from tearing itself apart as it rotates.
It is further believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with the Andromeda galaxy in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their super-massive black holes might even merge.
The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster – a mass concentration of galaxies that contains at least 100 galaxy groups and clusters within its diameter of 33 megaparsecs (110 million light-years).
You might be amazed to know that dung beetles actually navigate at night using the Milky Way. If you’ve never seen the Milky Way with your own eyes, you should take the chance when you can. Go to a place with nice dark skies, free from light pollution, and look up and appreciate the Milky Way. And be sure to wave hello to all the neighboring stars who share our galaxy with us.
We have written many interesting articles about the Milky Way here at Universe Today. Here is a one that is full of Milky Way facts, as well as articles that ask How Big is the Milky Way?, why it is called the Milky Way, and why it rotates.
Astronomy Cast also offers a good podcast about the large scale make-up of the universe.